The present invention relates to a semiconductor device and a method of fabricating the semiconductor device, and particularly to a semiconductor device in which solder bumps are formed on electrode pads via a barrier metal film and a method of fabricating the semiconductor device.
To increasingly progress miniaturization of electronic equipment, it is important how improve the mounting density of electronic parts. With respect to semiconductor ICs (Integrated Circuits), studies have been actively made to develop such a high density mounting technology as to directly mount a bare chip on a printed wiring board, typically a flip-chip mounting method in place of the conventional packaging method.
The flip-chip mounting method is represented by an Au (gold) stud bump method or a solder ball bump method, and further, it includes various other methods covering a wide range from those at the prototype stage to those at the commercially practical stage. The present applicant has developed in advance of time a mass-production technology for forming solder ball bumps on a wafer level including a number of chips by applying the LSI (Large-Scale Integrated Circuit) fabrication process, and has filed a number of the associated patent applications and simultaneously put consumer-oriented equipment produced on the basis of such a technology, for example, a micro digital camcorder, to thus early commercialize the next-generation LSI mounting technology.
In formation of solder ball bumps, a barrier metal film is provided between Al (aluminum) based electrode pads of an LSI and the bumps in order to improve the adhesion between the electrode pads and the bumps and prevent the mutual diffusion therebetween. In particular, for the solder ball bump method, since the barrier metal film exerts a large effect on the finished shapes of bumps, such a film is generally called a BLM (Ball Limiting Metal) film.
In general, the BLM film used for solder bumps has a three-layer structure of Cr (chromium) film/Cu (copper) film/Au film. In this three-layer structure, the bottom Cr film functions as an adhesion layer for ensuring good adhesion with Al electrode pads; the intermediate Cu film functions as a barrier layer for preventing the diffusion of solder from solder bumps; and the top Au film functions as an oxidation preventive film for preventing the oxidation of the Cu film.
A related art method of forming solder bumps using such a BLM film will be described below with reference to FIGS. 8 to 12.
First, an Al based electrode pad 32 made from Al or an Alxe2x80x94Cu alloy is formed on a connection of a flip-chip IC (not shown) formed on the surface of a semiconductor substrate 30. The entire surface of the substrate is covered with a passivation film (surface protective film) 34 made from polyimide film or silicon nitride film, and a BLM film 36 is formed in such a manner as to be connected to the Al based electrode pad 32 via a connection hole opened in the passivation film 34 (see FIG. 8).
The entire surface of the substrate is coated with a sufficiently thick photoresist film 38, and the photoresist film 38 is patterned by photolithography to form an opening 40 having a diameter being large enough to expose the BLM film 36 and a portion of the passivation film 34 around the BLM film 36 (see FIG. 9).
A solder vapor-deposition film 42 made from Pb (lead) and Sn (tin) is formed over the entire surface of the substrate by use of, for example a vapor-deposition method. At this time, since the end, positioned at the edge of the opening 40, of the photoresist film 38 is largely stepped, the solder vapor-deposition film 42 is divided into a solder vapor-deposition film 42a located on both the BLM film 36 and the portion of the passivation film 34 around the BLM film 34 in the opening 40 and a solder vapor-deposition film 42b located on the photoresist film 38 (see FIG. 10).
The solder vapor-deposition film 42b located on the photoresist film 38 is removed together with the photoresist film 38 by a lift-off method. The lift-off method is performed by dipping the wafer in a resist separation solution and heating/oscillating the wafer in the solution, to lift-off the photoresist film 38. In this way, only the solder vapor-deposition film 42a, which covers the BLM film 36 and the portion of the passivation film 34 around the BLM film 36, remains (see FIG. 11).
The solder vapor-deposition film 42a is then subjected to wet-back treatment. That is to say, the solder vapor-deposition film 42a is coated with flux and is fused by heat-treatment, to finally form a solder ball bump 44 connected to the BLM film 36. In this way, a semiconductor device is fabricated, in which the solder ball bumps 44 are formed via the BLM film 36 on the Al based electrode pads 32 formed on the connections of the flip-chip IC formed on the surface of the semiconductor substrate 30 (see FIG. 12).
While Al has been used as an interconnection material of LSIs for a long time, the use of Al has come to cause serious problems associated with delay of signals or deterioration in the reliability in multiple interconnection layers along with tendency toward higher integration, finer-geometries, and higher operational speed of LSIs.
In place of such a conventional material Al, Cu (copper) is expected as an interconnection material of the next-generation LSIs and is nearing practical use. Cu has a resistivity lower about 40% than that of Al (the resistivity of Al is about 2.8 xcexcxcexa9cm while the resistivity of Cu is about 1.7 xcexcxcexa9cm), and further Cu has a high resistance against electromigration. Accordingly, the use of Cu is expected to further reduce the resistance of the LSIs and improve the reliability thereof.
However, since the above-described process of forming solder ball bumps on electrode pads via a BLM film is predicated on the use of Al based electrode pads, if such a process is applied to an LSI using Cu electrode pads as terminals of Cu interconnections in place of the conventional Al based electrode pads as terminals of Al based interconnections, there may occur the following problem. Namely, since the adhesion strength between the BLM film and the Cu electrode pads is lowered, there may easily arise the falling of solder ball bumps from a semiconductor chip upon mounting of the semiconductor chip on a printed wiring board or the failure in electric contact characteristic between the Cu electrode pads and the solder ball bumps when the semiconductor chip undergoes a temperature cycle or a high temperature load, thereby exerting adverse effect on the device reliability.
Consequently, it is desired to establish a fabrication process of stably forming solder ball bumps with a high-reliability to the next-generation LSIs adopting a Cu interconnection material.
An object of the present invention is to provide a semiconductor device with a high-reliability and a high-durability, which is capable of forming solder bumps on electrode pads made from a metal other than Al without degrading the adhesion strength between the electrode pads and the solder bumps and the electric contact characteristic therebetween, and to provide a method of fabricating the semiconductor device.
To achieve the above object, according to a first aspect of the present invention, there is provided a semiconductor device including: electrode pads provided on a base; and solder bumps formed on the electrode pads via a barrier metal film; wherein an adhesion layer is formed between the electrode pads and the barrier metal film for increasing the adhesion therebetween.
The electrode pads is preferably made from Cu or an alloy containing Cu (hereinafter, these pads are referred to as xe2x80x9cCu based electrode padsxe2x80x9d).
The adhesion layer is preferably made from at least one kind of metal selected from a group consisting of Al, Ti (titanium), Cr, Co (cobalt), Ni (nickel), Mo (molybdenum), Ag (silver), Ta (tantalum), W (tungsten) and Au, or an alloy containing the at least one kind of metal.
In the semiconductor device according to the first aspect of the present invention, since the adhesion layer is formed between the electrode pads and the barrier metal for increasing the adhesion therebetween, even if the electrode pads are changed from the convention Al based electrode pads into the Cu based electrode pads, it is possible to prevent degradation of the adhesion strength between the Cu based electrode pads and the solder bumps and the failure of the electric contact characteristic therebetween. That is to say, in order to keep up with the next-generation high speed LSIs adopting Cu based interconnections, the function of the barrier metal film having been used for improving the adhesion between electrode pads and solder bumps is reinforced by provision of the adhesion layer.
Accordingly, the solder ball bumps appropriate to the next-generation high speed LSI adopting the Cu interconnection layer can be formed, and since the barrier metal function is reinforced by adding the adhesion layer to the conventional barrier metal film, even if various heat-treatments are applied to the substrate after formation of the solder layer, it is possible to effectively prevent thermal diffusion of solder, and hence to obtain good electric contact characteristic between the finally formed solder bumps and the Cu based electrode pads and also increase the adhesion strength therebetween. This makes it possible to improve the reliability and durability of a device product on which the semiconductor chip is mounted by the flip-chip mounting method. In summary, according to the semiconductor device of the first aspect of the present invention, it is possible to improve the electric contact characteristic, reliability, and durability of a device product on which the next-generation high speed LSI chip adopting Cu interconnections is mounted by the flip-chip mounting method.
According to a second aspect of the present invention, there is provided a method of fabricating a semiconductor device in which solder bumps are formed on electrode pads provided on a base via a barrier metal film, the method including: a first step of forming a passivation film in such a manner as to cover electrode pads; forming a resist film on the passivation film and patterning the resist film into a specific shape; and selectively etching the passivation film using the resist film as a mask to expose the electrode pads; a second step of forming an adhesion film over the entire surface of the base; and removing a first portion, located on the resist film, of the adhesion film together with the resist film by a lift-off method, to allow only a second portion, located on the electrode pads, of the adhesion layer to remain; and a third step of forming solder bumps on the second portion of the adhesion layer located on the electrode pads via a barrier metal film.
The second step preferably includes a step of forming the adhesion layer over the entire surface of the base by a sputtering method, an electrolytic plating method, or a CVD (Chemical Vapor Deposition) method.
In the method of fabricating a semiconductor device according to the second aspect of the present invention, the passivation film is selectively etched using the resist film patterned into a specific shape as a mask to expose the electrode pads, and of the adhesion layer formed over the entire surface of the base, the unnecessary portion of the adhesion layer located on the resist film is removed together with the resist film by lifting-off the resist film, to allow only the portion of the adhesion layer located on the electrode pads to remain. That is to say, the resist film patterned into a specific shape is used for both the etching mask and the lift-off film. Accordingly, it is possible to eliminate the necessity for provision of the step of forming a resist film for forming the adhesion layer only on the electrode pads and the lithography step for patterning the resist film, and hence to effectively form the adhesion layer in self-alignment over the entire surfaces of the electrode pads exposed by selective etching of the passivation film without increasing the number of processing steps.
Accordingly, even if the electrode pads are changed from the Al based electrode pads into the Cu based electrode pads, the adhesion layer can be effectively formed in self-alignment on the entire surfaces of the Cu based electrode pads. This makes it possible to increase the adhesion between the Cu based electrode pads and the barrier metal film and hence to prevent the degradation of adhesion strength between the Cu based electrode pads and the solder bumps and the failure in electric contact characteristic therebetween. In summary, according to the method of fabricating the semiconductor device of the second aspect of the present invention, it is possible to improve the electric contact characteristic, reliability, and durability of a device product on which the next-generation high speed LSI chip adopting Cu interconnections is mounted by the flip-chip mounting method.
While description has been made of the case adopting a Cu based material as the interconnection material of the next-generation high speed LSI, the present invention is not limited thereto. Even if a metal other than Cu is adopted as the interconnection material, the present invention can keep up with the future LSIs by selecting the material of the adhesion layer for increasing the adhesion between electrode pads and a barrier metal film in consideration of the new interconnection material other than Cu.
In this way, the semiconductor device and the fabrication method thereof according to the present invention is very useful to realize a semiconductor device designed on the basis of the fine design rule and required having high-integration, high-performance, and high-reliability.